A compressor consist of a series of rotating or stationary blade rows in which the combination of a rotor (circular rotating blade row) and a stator (circular stationary blade row) forms one stage. Inside the rotor, kinetic energy is transferred to the flowing gas (usually air) by the individual airfoil blades. In the following stator, this energy is manifested as a pressure rise in the gaseous air as a consequence of deceleration of the gaseous flowing air. This deceleration of the gaseous flowing air is induced as a result of the design of the stator section. The pressure ratio (exit pressure/inlet pressure) of a single stage is limited because of intrinsic aerodynamic factors, so several stages are connected together in many turbo compressors to achieve higher pressure ratios than could be achieved by a single stage.
When operating an axial compressor, mechanical vibrations of compressor components, especially of rotor and stator blades, occur. An important cause for these mechanical vibrations are pressure fluctuations within the compressor stages.
The air fluid flow around each blade has an associated boundary layer which covers each blade and coheres to the blade in case of the rotor blades. The flow boundary layer associated with a rotor blade will rotate as an associated entity of the blade as the blade itself rotates. At the downstream edge of each blade, this flow boundary layer melds into an associated flow boundary entity known as, alternatively, the Dellenregion, wake region, or delve region which is characterized by a localized reduction in both pressure and flow velocity. Therefore, each rotor produces at its downstream end a region with periodically changed flow and pressure characteristics at a characteristic frequency. This characteristic frequency is the product of the number of rotor blades and the present rotational speed. The frequency of these rotor-induced pressure fluctuations therefore depends on the rotational speed. The pressure fluctuations of the rotor of one stage interfere with the pressure fluctuations of the rotors of the neighboring stages. When these different rotors have a respectively different number of blades, interference pressure fluctuations are produced with an interference frequency being either the difference or the sum of the characteristic frequencies of these rotors involved.
While both the stator blades and the rotor blades in the compressor are subject to damage from forces associated with vibrational excitation, the rotor blades are especially at risk because of the additional centrifugal forces that can interrelate with those forces caused by vibration. Depending on the blade constructions, at least one resonance frequency of vibrational blade excitation lies between 100 and 1000 Hz. The aforementioned characteristic frequencies will be more than 4000 Hz. when the compressor is operating at its operational rotation rate; the interference pressure fluctuations, however, have 30 frequencies lying between 100 and 2000 Hz. too. Thus, there is the danger that, even in case of very slight variations of the compressor rotational speed during normal operation, the blades will be influenced to vibrate with their basic resonance frequency by the interference pressure fluctuations. The rotor or stator blades may then suffer damage or even break.
Contemporary turbo engines are usually equipped with fuel or energy control systems which measure and output a variety of operating parameters for the overall engine. Included in such control systems are highly accurate pressure sensing devices or systems. For example, pressure measuring systems are described in U.S. Pat. No. 4,322,977 entitled "Pressure Measuring System", filed May 27,1980 in the names of Robert C. Shell, et al; U.S. Pat. No. 4,434,664 issued Mar. 6, 1984, entitled "Pressure Ratio Measurement System", in the names of Frank J. Antonazzi, et al.; U.S. Pat. No. 4,422,335 issued Dec. 27, 1983, entitled "Pressure Transducer" to Ohnesorge, et al.; U.S. Pat. No. 4,449,409, issued May 22, 1984, entitled "Pressure Measurement System With A Constant Settlement Time", in the name of Frank J. Antonazzi; U.S. Pat. No. 4,457,179, issued Jul. 3, 1984, entitled "Differential Pressure Measuring System", in the names of Frank J. Antonazzi, et al.; and U.S. Pat. No. 4,422,125 issued Dec. 20, 1983, entitled "Pressure Transducer With An Invariable Reference Capacitor", in the names of Frank J. Antonazzi, et al.
While a wide variety of pressure measuring devices can be used in conjunction with the present invention, the disclosures of the above-identified patents are hereby expressly incorporated by reference herein for a full and complete understanding of the operation of the invention.
German Pat. Publication (Auslegeschrift) 2049338 discloses the detection of mechanical vibrations of rotor blades. An electromagnetic sensor mounted to the compressor housing detects the passage of rotor blades by magnetic induction. In case of blade vibrations, the signal output of the sensor is a periodic wave with the characteristic frequency, but modulated by the vibrational frequency of the blades. An electronic circuit extracts the modulation wave form from the sensor signal. By this known process, the actual induced mechanical vibrations of a single stage are measured. In cases of a highly resonant vibration excitation with a rapidly increasing vibration amplitude of the blade vibration, it is important that an eventual vibration excitation is detected as early as possible in order to take countermeasures in time. The known method of relying on the detection of mechanical vibrations is not suited to the challenge of early detection of unacceptable conditions since the fundamental causes of the vibrations can be in existence for some time before mechanical vibrations are detectable.